This happens most likely because of DPI awareness. Even if you are using WinForms platform, when LightningChart is created it loads some WPF libraries, and because of this the DPI awareness of the application is automatically modified. It changes to the WPF default of ‘system aware’, and the DIP-PX factor is changed to what is defined by windows settings. If your monitor’s Scale/zoom factor is >100%, then reduction of app window will be visible.

In short, you need to create an application manifest file and set <dpiAware>true</dpiAware>.
Then add the manifest toyour C# application by following these steps:
a. Right-click on your project in the Solution Explorer.
b. Select "Add New Item" from the context menu.
c. Choose "Application Manifest File" from the list of options in the dialog box that appears.
d. Next open ‘app.manifest’ file, search for ‘dpiAware’ string, uncomment relevant part and set DPI-awareness in a way you want.